The present invention relates to piston rings and, more particularly, to multiple-piece oil control rings.
Oil control rings are used in internal combustion engines to limit the flow of lubricant along the cylinder wall and into the combustion chamber. Excess amounts of lubricant are distributed onto the wall of the cylinder during engine operation to lubricate the pistons and cylinder wall, clean the cylinder wall, cool the cylinder wall and pistons and to increase the effectiveness of the seal between the piston rings and the cylinder wall. Oil control rings are necessary for acceptable oil economy and to control exhaust emissions.
Heretofore, a wide variety of piston ring configurations have been proposed for oil control purposes. The various proposals have included one-piece, two-piece and three-piece configurations. Each of the configurations typically includes an upper rail structure and an axially spaced lower rail structure, both of which engage the cylinder wall. Three-piece oil control rings include split or parted top and bottom rails and an annular spacer-expander which axially spaces the rails and biases them radially outward and into engagement with the cylinder wall. Examples of prior oil control rings may be found in U.S. Pat. No. 2,656,230 entitled PISTON RING ASSEMBLY AND ELEMENTS THEREOF, which issued on Oct. 20, 1953 to Phillips; U.S. Pat. No. 2,907,101 entitled METHOD OF PRODUCING A CONTINUOUS SPACER AND EXPANDER FOR PISTON RINGS, which issued on Oct. 6, 1959 to Hamm; U.S. Pat. No. 3,814,444 entitled LOW FRICTION PISTON RING, which issued on Jun. 4, 1974 to Johnson et al; and U.S. Pat. No. 4,194,747 entitled THREE-PIECE OIL CONTROL RING, which issued on Mar. 25, 1980 to Nisper.
One known type of spacer-expander used in a three-piece oil control ring is formed from a strip of steel as a series of alternating corrugations or loops. Each loop is generally symmetrical about an axial centerline. Each loop or corrugation includes a pair of angled legs joined to a pad portion. A tab or lug is formed along an inner circumference of the spacer-expander. The tab, which may be straight or angled, engages an inner circumferential surface of the split rail.
Secondary wear can occur at the interface between the inside diameter of the rail and the tab or lug that contacts the rail. Excessive wear at this interface or contact line may result in a loss of radial thrust or in so-called unitizing of the rail at the contact line. The wear, therefore, can result in a loss of oil control ring assembly function. Attempts to overcome this problem in the past have included chrome plating the inside diameter of the rail, chrome plating of the spacer-expander, nitriding or the use of alternate, more costly materials.
Current engine design has increased this secondary wear problem. Current internal combustion engines have been incorporating pistons with reduced ring groove width due to a reduction in overall piston height. With conventional corrugated spacer-expanders, a reduction in the overall height of the spacer-expander due to a reduction in the axial width of the groove reduces the length of the surface in contact with the rail. This reduction in contact length results in an increase of unit pressure. The increase in unit pressure exacerbates the secondary wear problem.
Other problems encountered with current oil control rings include localized groove side wear. This results from an increase in the pressure towards the sides of the piston ring groove at the radial position of the spacer gap. Problems have been experienced with conformation. Forming problems can cause the spacer to appear nonsymmetrical and, hence, have poor conformation or shape. In addition, the formation steps may produce a spacer-expander with unbalanced residual stresses resulting in inconsistency in part formation.